1. Field of the Invention
The invention relates to the manipulation of magnetic fields and more particularly to making magnetic fields that rotate or oscillate about an axis. Such magnetic fields are employed in industrial processes that involve ionized atoms or molecules to achieve better distribution of the ionized atoms or molecules with regard to a workpiece, typically a wafer.
2. Description of Related Art: FIGS. 1, 2, and 8
Many industrial processes involve the application of ionized atoms or molecules to a substrate, either to implant the atoms or molecules in the substrate, to add a layer of atoms or molecules to the substrate, or to etch the substrate. In one large class of such processes termed reactive ion etching, or RIE, the process is done in a plasma reactor. The plasma reactor includes a vacuum chamber and a source of ions, typically a gas which is introduced into the vacuum chamber. The substrate is placed in the chamber, the chamber is evacuated, the gas is introduced, and radio frequencies are used to excite the gas to the point where it becomes a plasma.
A problem in the design and operation of plasma reactors is obtaining an even distribution of the ions with regard to the substrate. The distribution should be even not only with regard to numbers of ions, but also with regard to the energy of the ions. The need for even distribution is particularly pressing in the semiconductor industry, since failure to achieve an even distribution with regard to a semiconductor wafer may cause the devices on significant portions of the wafer to become non uniform or even useless.
One common technique for improving the distribution of ions with regard to the substrate is what is termed magnetic enhanced reactive ion etching, or MERIE. In MERIE, a moving magnetic field is used to increase the uniformity of the plasma and prevent plasma drift. FIG. 1 is a schematic of a typical plasma reactor 101 with MERIE as seen from above. Only those details of system 101 that are relevant to MERIE are shown here. The wall of the vacuum chamber is shown at 103; in a MERIE system, the wall is of non-magnetic materials. Within chamber 103 is substrate 105 to which the ions in the chamber are to be applied. Surrounding chamber 103 in an axial manner with regard to chamber 103 are two pairs of coils 107, pair 107(1), which are labeled N for north and S for south, and pair 107(2), which are labeled E for east and W for west. The arrows 116 on the coils show the direction of current flow in the top leg of the coil. In the vertical legs, current that is moving towards the observer is indicated by xe2x80x9c.xe2x80x9d; current that is moving away from the observer is indicated by xe2x80x9cxxe2x80x9d. When a coil is energized, it produces a magnetic field whose axis is perpendicular to the plane of the coil, as shown at 119 for N coil 107(1). When the coils 103 are energized, a magnetic field is produced that passes through chamber 103. The direction of the magnetic field, shown by arrow 117, is determined by the directions and strengths of the magnetic fields produced by the coils 107; thus, the magnetic field can be made to move by varying the amount of current being provided to each of the coils 107. Thus each coil 107 is energized by a separate power supply, PS 115 and the power supplies operate in a phase relationship with each other. The power supplies may be low-frequency AC power supplies, or they may be power supplies that switch the DC current. The minimum number of power supplies is one for each opposing pair of coils 107, but 1 per coil is preferred, since such an arrangement provides closer control of the magnetic field inside the vacuum chamber. While most MERIE reactors use four coils as shown in FIG. 1, the technique can of course be extended to systems with more pairs of coils. For a detailed discussion of MERIE, see the section Background Art in U.S. Pat. No. 6,015,476, Schlueter, et al., Plasma reactor magnet with independently controllable parallel axial current-carrying elements, issued Jan. 18, 2000. U.S. Pat. No. 6,015,476 is hereby incorporated by reference in its entirety and for all purposes into present patent application.
FIG. 8 shows typical current curves 801 for PS 115(1-4). The current curve 803 for coil N 107(1) is 90xc2x0 out of phase with the current curve 807 for coil S 107(1), and the same is true for the current curve 805 for coil E 107(2) and the current curve 809 for coil W 107(2); moreover, current curves 805 and 809 are 45xc2x0 out of phase with current curves 803 and 807. The uniformity of the magnetic field produced by coils 107 varies during operation. Uniformity is worst at points such as point 813, where one pair of the coils 107 is receiving maximum current and the other is receiving minimum current and best at points such as point 811, where both pairs of coils are receiving the same amount of current. In plasma reactors of the type shown in FIG. 1, the uniformity of the magnetic field varies by 50% during a complete rotation of the magnetic field. This is shown in FIG. 2. The diagram labeled 811 shows the magnetic field at point 811 in the current curves of FIG. 8, namely when both coil pairs 107(1 and 2) are receiving the same amount of current; the diagram labeled 813 shows the magnetic field at point 813 in the current curves of FIG. 8, namely when one coil pair is operating at maximum current and the other at minimum current. As is clear from FIG. 2, the magnetic field at point 811 in the current curves is far more uniform than the magnetic field at point 813. The less uniformity there is in the magnetic field, the less uniformity there will be in the plasma in the neighborhood of wafer 105; consequently, the lack of uniformity shown at 813 in FIG. 2 represents a major problem in MERIE systems.
An important cause of the lack of uniformity in the magnetic field produced by MERIE system 101 is the corners 113 where the coils 107 approach each other. In these corners, the currents flowing in the adjacent vertical legs of the coils that meet at the corner interact with the magnetic field produced generally by the coils and with the local magnetic fields produced by the currents in the vertical legs. The effects of current flow in the legs are particularly strong when the current is flowing in the same direction in the adjacent legs, as in corner 113(2). Of course, because the magnetic field is rotating, each corner in turn becomes a corner like 113(2). In the following, the effects on the uniformity of the magnetic field caused by corners 113 will be termed corner effects.
Solutions have been proposed for improving the uniformity of the magnetic field in MERIE systems, but these solutions involve a complete redesign of the reactor and can thus only be implemented in new plasma reactors. For examples of these solutions see the Schlueter patent referenced above and U.S. Pat. No. 5,718,795, Plavidal, et al., Radial magnetic field enhancement for plasma processing, issued Feb. 17, 1998. What is needed, and what is provided by the present invention is low-cost solutions which can be easily retrofitted to existing plasma reactors.
The solutions provided by the invention include a magnetic shunt which may be retrofitted to a corner to compensate for corner effects, a trim coil which may be retrofitted and energized to compensate for the corner effects, a retrofittable combination of the shunt and the trim coil, and retrofittable coils which function as 180xc2x0 coils. The magnetic shunt, the trim coil, and the combination may all be employed to compensate for the remaining corner effects in systems using the retrofittable 180xc2x0 coils. A particularly advantageous combination of the trim coil and the magnetic shunt is used to deal with the fact that there are generally two levels of corner effects that must be compensated for. In this combination, the magnetic shunt is sized so that it compensates for the first level of corner effects and the energized trim coil and the magnetic shunt together compensate for the second level of corner effects. A further advantage of this arrangement is that the energized trim coil causes the magnetic shunt to saturate.
Other objects and advantages will be apparent to those skilled in the arts to which the invention pertains upon perusal of the following Detailed Description and drawing, wherein: